Lubricants are used in automotive, industrial, marine and aviation applications. They are used to decrease friction and wear, to protect from corrosion, to act as sealants and to affect heat transfer. Most of the lubricants are made from mineral oil and some are synthetic chemicals. Between 5,000 and 10,000 different lubricant formulations are necessary to satisfy more than 90% of all lubricant applications. About 8 million tons of lubricants are consumed every year world over. Out of this about 53% is, collected as waste, thus, endangering our planet. The use of rapidly degradable lubricants instead of the conventional petroleum-derived lubricants could significantly reduce this environmental pollution.
Plant-derived lubricants are renewable and biodegradable. Pure, natural lubricants manufactured by environmentally-safe procedures are gaining more attention in recent years, since they do not contain toxic compounds. Plant-based lubricants are presently used in some applications where there is an environmental risk, for example in marine, forestry and agricultural appliances. However, it is believed that around 90% of lubricants currently used could be replaced by plant-derived lubricants. Some of the advantages of plant-based lubricants include: (1) biodegradability and renewable nature, (2) excellent lubricity, lower friction coefficient than mineral oils, (3) lower evaporation up to 20% less than mineral oils, (4) higher flash point, reducing the risk of fires in applications such as metal cutting, (5) higher viscosity indices, and (6) enhanced performance in some applications. Several studies show that biolubricants have a longer lifespan than mineral oil lubricants. The high production cost of bio-lubricants (about 5 times more than the petrolubricants) is the main hurdle for their development at the current time. Development of eco-friendly and economically-beneficial routes for preparing plant or vegetable oil-based lubricant base oils is highly desirable.
Fatty acid polyol or long chain alcohol esters have the right composition to be used as lubricant base oils. They are miscible with hydrocarbons. U.S. Pat. No. 8,168,572 B2 discloses the use of the polyols esters in lubricant blend composition. U.S. Pat. No. 8,058,217 B2 teaches that the polyol esters are superior metal working fluids.
These fatty acid esters can be prepared by two different ways: (1) the direct esterification of fatty acids with polyol, and (2) transesterification of fatty acid methyl or ethyl esters (biodiesel) or vegetable oils with polyols. Conventionally, these esterification and transesterification reactions are catalyzed homogeneous mineral acid catalysts. U.S. Pat. No. 7,968,504 B2 teaches the method for preparing fatty acid esters by the reaction of fatty acid ester in the presence of homogeneous phosphoric acid catalyst, with a hydroxyl containing compound. The resulting product is useful as a lubricant, as a heat transfer agent, as a rheological modifier and as a corrosion/moisture inhibitor, among other uses. U.S. Pat. No. 8,101,560 B2 discloses the procedure for preparing lubricant base oil of palm origin, particulary fatty acid monoesters and fatty poly esters by esterifying palm fatty acid with a monohydric or polyhydric alcohol in the presence of a homogeneous acid catalyst.
While homogeneous base catalyst (alkali hydroxides and alkoxides) are efficient for transesterification, they are no good for esterification reactions as the base reacts with fatty acid and forms metallic sops (Masood et al., Appl. Catal. A: Gen., Vol. 425-426, Year 2012, pp. 184-190).
In general, corrosiveness, sensitivity to water, catalyst recovery, environmental hazards and waste control are the serious issues with the homogeneous mineral acid and alkali base catalysts. Solid catalysts have environmental and engineering advantages. They can be easily separated and reused.
References are made to Díaz et al., Micropor. Mesopor. Mater. Vol. 80, Year 2005, pp. 33-42; Pérez-Pariente et al., Appl. Catal. A: Gen. Vol. 254, Year 2003, pp. 173-188; Bossaert et al., J. Catal. Vol. 182, Year 1999, pp. 156-164; and Pouilloux et al., J. Mol. Catal. A: Chem. Vol. 149, Year 1999, pp. 243-254 which disclose the use of zeolites, ion-exchange resins and metal ion-exchanged- or sulfonic acid-functionalized ordered mesoporous silica materials as solid acid catalysts. However, pore-size limitation, loss of activity in presence of by-product water and formation of undesired products are some issues with these solid catalysts. Enzyme catalysts are selective but require longer reaction times (48 hrs and more).
U.S. Pat. No. 7,842,653 and EP 1733788 disclose the use of solid, acid, double metal cyanide catalyst for preparation of lubricants by reacting vegetable oil or fat obtained from animal source with an alcohol at a temperature in the range of 150° C. to 200° C. for a period of 3 to 6 h. Conversion of triglycerides into glycerol in the range 90 to 96 mol % was obtained. Product of this reaction is fatty acid alkyl ester and glycerol. The alcohol used is a normal or branched alcohol, selected from the group consisting of hexanol, heptanol, octanol and their mixture thereof. The product lubricant obtained comprises of C22-C28 fatty acid alkyl esters. It doesn't disclose the application of this catalyst for reactions with polyols and reaction of fatty acids or fatty acid alkyl esters with mono- or polyhydric alcohols.
U.S. Pat. No. 8,124,801 teaches a process for preparation of fatty acid alkyl esters wherein the process includes contacting fatty acid glycerides with alcohols in the presence of a separable catalyst which includes a metal from Group VIB of the periodic table and an element from Group VA of the periodic table. The process comprises using at least one of the alcohols essentially selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, octanol, 2-ethylhexanol, decanol, dodecanol and mixture thereof. One of the alcohols has a carbon number ranging from 1 to 50. This process doesn't disclose specifically the use polyols and also conversion of fatty acids or fatty acid methyl or ethyl esters to lubricant base oils. Moreover, relatively long reaction times and high temperature is required.
Article titled, “Synthesis of biolubricants using sulfated zirconia catalysts” by Jinho Oha, Sungeun Yanga, Chanyeon Kima, Inchang Choib, Jae Hyun Kimb, Hyunjoo Leea in Applied Catalysis A: General 455 (2013) 164-171 reports the synthesis of biomass-derived lubricants via esterification, transesterification, and simultaneous reactions of both by using sulfated zirconia catalysts. Soybean oil or free fatty acids derived from soybean oil were used as a biomass-derived resource for the synthesis of biolubricants. Long chain alcohols (carbon number ≦8) or neo-polyols (e.g., 2,2-diethyl-1,3-propanediol, trimethylol propane, pentaery-thritol) were used as co-reactants. The paper further reports that the structure of the alcohol significantly affected the conversion and yield for the esterification with oleic acid.
WO 2012/114357 discloses a process for preparing polyesters by reacting polyols with polycarboxylic acid in presence of heterogeneous, reusable, acid, crystalline, micro-mesoporous double metal cyanide catalyst at moderate temperature and short period of time. Polyolester produced is a hyperbranched polymer having degree of branching in the range 45 to 90% and inherent viscosity in the range 0.02 to 0.1 dL/g.
In view of the importance of polyol and other synthetic esters in industrial applications and drawbacks of prior-art processes which include catalyst deactivation, formation of undesired products, high temperature requirement and long reaction times, it is desirable to have a more efficient, stable, solid catalyst-based esterification/transesterification process for fatty acid polyol or monool esters synthesis. The process of the present invention using solid phosphonate catalyst is highly efficient and overcomes the above-cited deficiencies of the prior-art processes.